Various devices have been previously developed for transforming linearly polarized waves into circularly polarized waves. Examples include waveguides equipped with corrugations, screw arrays, dielectric sheets and irises, either individually or in combination. Few of these devices, however, are capable of simultaneously transforming waves of two different frequency bands, and those that do have this capability suffer from one or more shortcomings (e.g., high manufacturing costs, low power handling, and difficult and expensive tuning).